Multi-hop packet radio networks

ABSTRACT

An adaptive communication system utilizes opportunistic peak-mode transmissions to transmit data between originating and destination stations, via one or more intermediate stations. Each station monitors the activity of other stations in the network, storing connectivity information for use in subsequent transmissions. Each station also sends out probe signals from time to time, to establish which other stations are in range. Messages are then sent across the network from station to station, with confirmation data being transmitted back to the originating station, until the destination station is reached. Old messages, which would otherwise clog the network, are timed out and deleted. A communication network and transceiver apparatus for use in the network are also disclosed.

BACKGROUND OF THE INVENTION

THIS invention relates to a method of transmitting data betweenoriginating and destination stations in a multi-station communicationnetwork, to a communication network for implementing the method, and tocommunication apparatus for use in the network.

Communication networks are known which require one or more controllingnodes or base stations through which messages must be routed fromoriginating to destination stations. Such networks are vulnerable tobreakdown of the controller nodes or base stations. In addition, thecontroller nodes or base stations are relatively expensive, and remotestations in the network are restricted in their movement with respect tothe base stations.

The connectivity between stations in such a network may change due torelative movement between remote stations and the base station,interference, noise and other factors. In a Rayleigh fading environment,the rate of fluctuation of signal strength, noise and interferencechanges the connectivity between stations in the network on aninstantaneous basis, making any method of fixed routing or adaptiverouting by the passing of routing information between stations almostimpossible. Generally, in order to compensate for interference andfading, messages are transmitted redundantly and with sufficient powerto ensure their reception, leading to sub-optimal utilisation of thenetwork and to interference between stations. Sub-optimal utilisation ofthe network results in a reduction in the network capacity (Erlangs) fora given area and a given spectrum allocation.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof transmitting message data from a first one of a plurality of stationsin a network to a second one of the plurality of stations, the methodcomprising:

monitoring, at the first station, the activity of other stations in thenetwork;

transmitting the message data to at least one opportunistically selectedintermediate station for onward transmission to the second station; and

transmitting confirmation data back from the at least one intermediatestation to the first station, indicative of the onward transmission ofthe message data.

Preferably, each station in the network monitors the activity of otherstations on an ongoing basis in order to determine the availability ofthose other stations, according to predetermined criteria, asintermediate or destination stations.

The monitoring may be carried out by receiving data transmitted by theother stations, and analysing the received data transmissions to selectan intermediate or destination station.

The monitoring may further include extracting information from thereceived data indicating at least the identity of the other stations.

For example, the information may relate to the destination and/or theorigin of message data transmitted to or received from the otherstations.

The method may also include extracting information from the receiveddata relating to the final destination and/or primary origin of themessage data.

The method may further include extracting information from the receiveddata relating to the propagation delay of each message, the data rate ofeach message and/or the volume of messages between any two or morestations.

The data transmitted by each station may include time data, themonitoring including determining the age of data transmissions receivedfrom other stations in the network and discarding data transmissionsolder than a predetermined age.

The time data in the received data transmissions may be compared with areference time, and the received data transmissions may be discarded apredetermined period after the reference time.

The method preferably includes allocating a priority to received datatransmissions, and adjusting the order of retransmission of the receiveddata transmission to other stations according to the age thereof.

The method may include monitoring the quality of the signal path betweenthe first station and one or more of the other stations and adapting,according to predetermined criteria, at least one parameter of asubsequent transmission to another station in accordance with themonitored quality of the signal path to increase the probability of thetransmission being received successfully.

Preferably, information is extracted from the received data relating tothe quality of the transmission path between any two or more of theother stations.

The method may include deriving from the received data adaptationinformation for use in adapting, according to at least one predeterminedcriterion, at least one parameter of a subsequent transmission toanother station to increase the probability of the transmission beingreceived successfully.

The adaptation information may be transmitted to one or more otherstations in an adaptation signal, the one or more other stations beingresponsive to the adaptation signal to vary at least one parameter of asubsequent transmission therefrom.

The parameter which is adapted may be one or more of the data rate,transmission power, transmission frequency, transmission or receptionantenna, message length, message priority, message time to live, time oftransmission, and message retransmission rate.

The monitoring step preferably further includes transmitting a probesignal from the first station to at least one intermediate station, theprobe signal containing at least address data identifying the firststation (and preferably the second station), and transmitting anacknowledgement signal from the selected intermediate station to thefirst station.

According to a second aspect of the invention there is provided acommunication network comprising a plurality of stations each able totransmit and receive message data, each station comprising:

transmitter means for transmitting data to other stations;

receiver means for receiving data from other stations;

monitoring means for monitoring at least one characteristic ofrespective channels between a first station and other stations;

decision means for selecting another station as an intermediate stationfor onward transmission of message data from the first station to adestination station; and

control means for adjusting at least one parameter of a transmissionsignal transmitted by the transmitter means according to the monitoredat least one characteristic of the respective channel to increase theprobability of the transmission signal being received successfully bythe selected intermediate station.

The monitoring means of each station is preferably adapted to analysedata in signals received from other stations to select the intermediatestation.

The control means is preferably adapted to monitor the age of datatransmissions received from other stations in the network and to discarddata transmissions older than a predetermined age.

The control means may be arranged to include time data in each datatransmission, to monitor the age of received data transmissions bycomparing time data therein with a reference time, and to discard thereceived data transmissions a predetermined period after the referencetime.

Preferably, the control means is arranged to allocate a priority toreceived data transmissions and to adjust the order of retransmission ofthe received data transmissions to other stations according to the agethereof.

Each station may include storage means for storing data in the receivedsignals relating to the identity of the other stations, and processormeans for determining the quality of the signal path between thereceiving station and each of the other stations.

The monitoring means is preferably adapted to generate a probe signalfor transmission to other stations, the probe signal containing at leastaddress data identifying the originating station (and preferably thedestination station); and to receive an acknowledgement signal fromother stations receiving the probe signal.

The control means is preferably adapted to vary the data rate,transmission power, transmission frequency, transmission or receptionantenna, message length, message priority, message time to live, time oftransmission, message retransmission rate, and/or other parameters ofits transmission to the selected intermediate station.

According to a third aspect of the invention there is providedcommunication apparatus for use as a station in a communication networkcomprising a plurality of stations each able to transmit and receivemessage data, the communication apparatus comprising:

transmitter means for transmitting data to other stations;

receiver means for receiving data from other stations;

monitoring means for monitoring at least one characteristic ofrespective channels between the apparatus, operating as a first station,and other stations;

decision means for selecting another station as an intermediate stationfor onward transmission of message data from the first station to adestination station; and

control means for adjusting at least one parameter of a transmissionsignal transmitted by the transmitter means according to the monitoredat least one characteristic of the respective channel to increase theprobability of the transmission signal being received successfully bythe selected intermediate station.

The monitoring means is preferably adapted to analyse data in signalsreceived from other stations to select the intermediate station.

The apparatus may include storage means for storing data in the receivedsignals relating to the identity of the other stations, and processormeans for determining the quality of the signal path between thereceiving station and each of the other stations.

The monitoring means is preferably adapted to generate a probe signalfor transmission to other stations, the probe signal containing at leastaddress data identifying the originating station (and preferably thedestination station); and to receive an acknowledgement signal fromother stations receiving the probe signal.

The monitoring means may be adapted to vary the data rate, transmissionpower, transmission frequency, transmission or reception antenna,message length, message priority, message time to live, time oftransmission, message retransmission rate, and/or other parameters ofits transmission to the selected intermediate station.

Preferably, the monitoring means comprises power sensing means andcontrollable attenuator means responsive to power control signalsderived from an output of the power sensing means to attenuate receivedand/or transmitted signals to within predetermined levels.

The controllable attenuator means may comprise a plurality of resistiveelements and a plurality of associated solid state switch elementsresponsive to the power control signals and arranged to connect theresistive elements to, or disconnect them from, the signal path.

The control means is preferably adapted to adjust the transmission powerof the transmission signal in response to the measured power of areceived signal.

The control means may include current or power sensing means formonitoring the transmission power of the transmission signal, comparisonmeans for comparing the transmission power with the measured power of areceived signal and for generating a transmission power control signal,and controllable driver means in the transmitter means responsive to thetransmission power control signal to adjust the transmission powertowards a value having a predetermined relationship with the measuredpower of the received signal.

The monitoring means preferably includes demodulator means operable at aplurality of predetermined data rates, thereby to demodulate receiveddata at any one of the predetermined data rates.

The demodulator means may comprise a plurality of demodulators arrangedin parallel and each operating at a respective different predetermineddata rate.

Preferably, the demodulator means further comprises selection means formonitoring the outputs of the parallel demodulators and for selecting anoutput which is delivering validly demodulated data.

The apparatus may include processor means and associated vocoder meansfor converting speech to data for transmission and for convertingreceived data to speech.

The vocoder means preferably comprises at least two vocoders arranged inparallel and operable at different data rates, the processor means beingoperable to select data from the vocoders for transmission according tothe monitored at least one characteristic of the channel.

The at least two vocoders are preferably operable independently toconvert a speech signal to respective different data signals atdifferent data rates or using different vox settings, the processormeans being operable to select any one of the different data signals fortransmission.

The processor means may be operable to output received data to aselected one or more of the vocoders at a rate selected to convert thereceived data to speech according to predetermined criteria.

The processor means may also be operable to add or remove dataselectively from the received data output to the selected one or more ofthe vocoders to control the rate at which a speech signal represented bythe received data is replayed.

In a preferred embodiment, the at least two vocoders are operableindependently, at least one to convert a speech signal to data fortransmission, and at least one to simultaneously convert received datato speech.

The control means is preferably adapted to monitor the age of datatransmissions received from other stations in the network and to discarddata transmissions older than a predetermined age.

The control means may be arranged to include time data in each datatransmission, to monitor the age of received data transmissions bycomparing time data therein with a reference time, and to discard thereceived data transmissions a predetermined period after the referencetime.

Preferably, the control means is arranged to allocate a priority toreceived data transmissions and to adjust the order of retransmission ofthe received data transmissions to other stations according to the agethereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block schematic diagram illustrating the hardwareof a single station of a communication network according to theinvention;

FIG. 2 is a simplified schematic diagram illustrating communicationbetween originating and destination stations in the network;

FIG. 3 is a state diagram illustrating a typical decision making processemployed by the stations of the network;

FIG. 4(a) show a flow diagram of a routing decision algorithm and 4(b)employed by the stations of the network;

FIG. 5 is an example of a typical data message structure employed by thenetwork;

FIG. 6 is an example of a typical probe/acknowledgment message structureemployed by the network;

FIG. 7 is a diagram showing message flow in the network;

FIG. 8 is a schematic block diagram of a transmitter module of atransceiver according to the invention;

FIG. 9 is a schematic block diagram of a receiver module of thetransceiver;

FIG. 10 is a schematic block diagram of a main processor and modeminterface module of the transceiver;

FIG. 11 is a schematic block diagram showing the main processor togetherwith a dual vocoder interface module of the transceiver;

FIG. 12 is a schematic diagram of a multi-stage switched attenuator ofthe transceiver; and

FIG. 13 is a flow chart-type diagram giving an overview of the overallsoftware architecture of the transceiver

DESCRIPTION OF EMBODIMENTS

The present invention has primary application in wireless data networks,including mobile radio or cellular telephone networks, two-way pagingnetworks, meteor burst PCN data networks and low earth orbiting andgeostationary satellite environments, where rapidly and greatly changingconnectivity and platform population changes militate against the use ofconventional networking techniques.

To this end, the invention provides a communication network which makesuse of adaptive opportunistic communication between stations in thenetwork. The network is a full mesh network which accommodates rapidlychanging connectivity between stations and routes messages dynamicallybetween stations on a co-operative basis to improve data throughput inthe network, while minimizing power consumption and interference betweenstations. The invention optimises the network capacity by ensuringoptimal utilisation of the available spectrum in terms of capacity(Erlangs) for a given area, a given spectrum allocation and a giveninfrastructure cost (Erlangs/km² /Hz/$).

Referring first to FIG. 1, a single network station is shownschematically in a highly simplified form. It will be appreciated thatthe network stations may be portable transceivers or fixed stations, ora combination thereof.

The heart of the station is a microprocessor-based controller 10 whichoperates under the control of stored software, which derives informationby monitoring transmissions from other stations, both on an ongoingbasis and in response to specific probe signals transmitted by thestation. The station has one or more transmitting/receiving antennas 12which are connected via a combiner unit 14 to an adaptive receiver 16and an adaptive power amplifier/transmitter 18, all of which arecontrolled by the controller 10. Data is passed between the controller10 and the receiver 16 and the transmitter 18 via an adaptive data ratemodem 20. An input circuit 22 receives, for example, voice, data and/orvideo signals and includes analogue to digital converters withassociated adaptive encoding processors, under the control of thecontroller 10, for converting the signals to a digital format andfeeding them to the controller 10.

The controller 10 of each station continually analyzes data receivedfrom other stations which are within range, arising from theircommunication and interaction from time to time. The address informationfor other stations obtained in this way is collated and stored, andtranslated into connectivity information. The controller waits andmonitors the activity of the other stations, seeking an opportunityeither to communicate a message, as an originating station, or to relaya message to another station on behalf of a different originatingstation. When the controller detects a quiet time on the network, ittransmits a probe signal which contains its own address and the addressof the destination station, inter alia.

On receiving an acknowledgement signal from another station which isable to act as an intermediate or relay station, the controller thentransmits a burst of data containing a message (either an originalmessage or a relayed message). The transmission power, data rate,message lengths, message priority, message time to live, messageretransmission rate and other parameters are controlled according toinformation derived from the acknowledged signal, relating to thecharacteristics of the channel or link between the stations at thattime. The timing of the transmission is selected to take advantage ofpeaks in the signal strength or signal-to-noise levels typicallyexperienced in a Rayleigh fading environment, so that the mode ofcommunication is effectively an opportunistic time division multipleaccess system. By operating in peak mode, the required transmissionpower is reduced, reducing interference between stations, and reducingthe necessity for the retransmission of messages.

The existence of the abovementioned peaks may be due, for example, tovariations in signal path amplitude, frequency or phase variation, noiseor interference, multipath effects, etc. The occurrence of peaks can bedetected by monitoring physical characteristics of received signals orby monitoring bit error rates as a function of time.

(The design and operation of an individual transceiver apparatusoperable as a station in the above described manner is described ingreater detail below, with reference to FIGS. 8 to 13.)

The state diagram of FIG. 3 and the flow diagrams of FIGS. 4a and 4billustrate the operation of each station in the network. The statediagram of FIG. 3 illustrates the overall operation of a station, whilethe flow diagrams of FIGS. 4a and 4b illustrate a typical opportunisticmessage transmission procedure.

A key feature of the described system is the continual monitoring byeach station of the activity of other stations in the network, both fromthe point of view of selecting an optimal channel for each transmission,and for selecting a station to which a message is to be transmitted.Each message transmitted over the network, whether it is a data messageas illustrated in FIG. 5 or a probe/acknowledge message as illustratedin FIG. 6, contains its originating address, its destination address,and the address of the station relaying the message. Therefore, anyother station monitoring the channel will hear which other stations aretransmitting information, and which stations are relaying thatinformation.

As messages pass from station to station, the originating anddestination addresses in each message remain the same, but theintermediate address will be the address of the relaying station beingused for the next "hop". As each station receives a message, it analysesthe information it has derived from the channel and the stations aroundit and will then opportunistically, via a probe/acknowledge cycle,choose another intermediate station address, to relay the messageonwardly via that station. Obviously, an originating station and adestination station may be able to communicate directly. However, inmany cases the originating station will not be able to communicatedirectly with the destination station, but will be able to transmit itsmessage to an intermediate station which it has heard talking to thedestination station, either directly or via one or more additionalintermediate stations. Each time a message is sent to an intermediatestation which cannot directly communicate with the destination station,it will seek a further intermediate station that is in communicationwith the destination station or that it has heard talking to thedestination station.

If the intermediate station cannot reach the destination station throughany route (i.e. it has received no information from other stationscontaining the address of the destination station) it will immediatelyrevert back to the previous station, so that the station can attempt tofind another route for transmission of its message.

It will be apparent that there are at least two different message typesbeing transmitted over the network: a probe/acknowledgement message anda data message. The probe/acknowledgement messages are used primarilyfor control and feedback adaption, while the data messages are used forinformation/message transmission across the network. The data messagecan use any data rate, while the probe/acknowledgement revert backmessages normally use a standard network data rate. However,probe/acknowledgement messages can be sent at different data rates,allowing the stations to establish what rate is best for the particularcircumstances.

Referring now to FIG. 3, a typical opportunistic message send flow modeis illustrated. In block A, a message is entered or received for relay.The controller 10 then goes to block B where it determines the message'spriority depending on the elapsed time since the message wasentered/received, the priority attributed to other messages in itsstack, and the opportunities it has, based upon the historical datadeveloped in block J, to send the message. It then examines theinformation based upon network statistics and monitoring and establishesits message priority with respect to other message priorities on thenetwork, taking into account network traffic. It then makes the decisionas to whether it should monitor and wait to hear the destinationstation, or a station talking to the destination station, or whether itshould enquire or probe to find the destination station or a relay(intermediate station) to the destination station. If the message is oflow priority, the controller will go to block D and wait in monitoringmode for a period of time to hear either the destination station itself,or a relay talking to the destination station. Using this method, asingle or double hop route is found.

Should the time set based upon the priority of that message elapse, thecontroller proceeds to block E where it waits for an opportunity totransmit by waiting for a break in network activity and for channelconditions to be suitable, and then enquires or probes for thedestination address in block H. If the destination station responds, thecontroller proceeds to block I where it will, based on adaptivefeedback, send a message to the destination station. The controller thenreturns back to the monitoring mode in block D.

Should the probe signal not receive an acknowledgement from thedestination station, the controller will then probe for an opportunityto transmit via a relay or intermediate station (block G), and based onadaptive feedback from any such relay, send the message via that relay(block F). Depending on the results of the revert back message from thatrelay, the controller will return to the monitoring mode of block D.

When a station probes, it can also probe for any one of a particulargroup of stations, or probe for a station that has "heard" a particularstation or a station that has "heard" a transmission to a particularstation. Thus, probing can be used to locate other stations, or to findopportunities to communicate with other stations.

When the message is successfully transmitted onwards by the relay orintermediate station, it sends a revert back signal, together withfurther adaptive information, to the originating station. Theoriginating station then returns to the monitor mode of block D andwaits for an end-to-end message acknowledgement, as well as any furtherrequests for relaying messages or receipt of messages. When a messagereaches its final destination, the destination station will send back amessage acknowledgement which might follow, due to the adaptiveopportunistic operation of the network, a completely different routeback to the originating station.

The probe/revert back messages employed by the network are used by thestations in an interactive manner in an analogous way to human beingscommunicating by "knocking" to attract each other's attention, "nodding"to indicate successful communication, and other adaptive interaction tomodify the various parameters of their communication.

It will be appreciated that the stations of the network are not arrangedin a hierarchy with controllers, base stations or parent stations. Thenetwork hierarchy is fully distributed and only message priority andtransmission opportunities dictate message flow. Thus, the stations ofthe network work co-operatively to maximise the overall throughput ofthe network.

One of the objectives of the overall network is to co-operativelymaximize the capacity in terms of Erlangs for a given spectrumallocation, a given area and a given equipment infrastructureinvestment. In other words, an objective of the network it to maximizeErlangs/km² /Hz/$.

It will be noted that no dedicated routing information is passed acrossthe network within either the data message or probe/acknowledge messagestructure, since no specific routing information is required for data toflow across the network.

Under normal conditions, the controller of each station will reside ateither block D or Block H of the state diagram of FIG. 3. In otherwords, it will either be monitoring or waiting for transmissionopportunities in a high activity network, or adaptively enquiring andreceiving occasional feedback in a low activity network. The only reasonfor enquiries is to create message activity or to find a particulardestination or relay opportunity. In a high-traffic situation, stationswill not probe but will rely on hearing other stations communicatingwith each other, thereby deriving connectivity and routing information.Thus, normal operation will usually comprise waiting for a destinationstation to communicate and then instantaneously and opportunisticallysending a message, either to that destination station directly or to anintermediate station talking to the destination station.

The described communication network has a number of specificcharacteristics:

1. The network allows any station to enter the network without the needto update network lists or to transfer network information.

2. Stations are able to adapt each other dynamically in a co-operativefashion so as to maximise message throughput and minimize contentionbetween stations.

3. Stations are able to probe and request a channel between them inorder to find opportunities of connectivity.

4. Stations are able to use revert back check acknowledgements, therebydynamically adapting each other's operating parameters and informingother stations as to the status of message flow (e.g. message arrival,requests for retransmission of messages, etc).

5. Stations are able to learn and form knowledge bases which allow themto make an optimal first try at sending a message across the network,based upon monitored information and feedback from other stations.

6. Messages are not sent in a rigid fashion. In the absence of a messageacknowledgement, messages will be resent. Messages that are "stuck" inthe network will "time out" after a predetermined period. Message time(duration), time to live, and time of creation are embedded in themessages. This allows the transmission of time-sensitive data to beaccelerated through the network depending upon its remaining time tolive and also allows time-sensitive data (such as voice data) which isno longer relevant to be timed out.

7. A station has an "intelligent" stack of network messages. When anyparticular station is heard, a suitable message can be drawn from thestack dynamically and sent to that particular station, to make maximumuse of transmission opportunities. Thus, where a station is routingmessages on behalf of a number of different stations through a number ofdifferent other stations, it can opportunistically combine messages andforward them to other stations, reducing overhead on the network.

8. Each station can monitor link or channel quality in terms of signalstrength, interference, signal-to-noise ratio, spike noise, etc., inorder to find the best opportunity to send messages during periods ofrelative quietness and optimal signal strength.

9. Stations have the ability to transmit the minimum required powerlevel required to reach the destination station or an intermediatestation, thereby minimising interference to other stations. Thetransmission power is adapted on a transmission by transmission basis,and is increased or decreased based upon information contained in revertback check signals from other stations. Routing and relaying of signalsis optimised so as to minimize stations transmitting at high power,thereby minimising interference and power consumption.

10. Stations always attempt to transmit using channel peaks, based uponavailability, using reduced signal strength and further minimisinginterference with other stations.

The described communication network has a number of advantages comparedwith prior art systems. For example, if a station finds itself in a highnoise environment, it may relay a message to an adjacent station,outside the noise environment, which can then in turn relay the messageto the destination station. Alternatively, stations in a highlycongested environment can reduce their power levels so as to effectivelyminimise interference, and relay messages amongst each other at lowpower and high data rates, thereby effectively using less time on theoverall network. The network can be interfaced transparently to aconventional multi-hop stable routing network with completetransparency. For example, should more than the typical 3 hops berequired to transmit a message from the originating station to thedestination station, the message can be routed to the fixed networkwhich uses conventional routing. The final 3 hops can again take placein the above described dynamic network.

In the described network, messages are routed "towards" their finaldestination one relay hop at a time, since each station is developingrouting information for every destination and the originating stationneed not rely solely on its own information to determine the messageroute. Since, in many cases, the last hop to the destination station isthe most difficult, the message may follow a number of extra hops toreach its final destination. Therefore, even with a nominal 3 hopsystem, since 3 hops are available at each intermediate station, 10 hopsor more may be used to reach the destination station. The reason forthis is that at each intermediate station, a brand new decision is madeas to how to reach the destination station and, each time, 3 more hopsare available. No memory of previous hops is kept, apart from theoriginating address. This method emphasises the importance of an"end-to-end" message acknowledgment, since in some cases the message mayreach an effective dead end where there is no possibility of theintermediate station concerned hearing the final destination through amaximum of 3 further intermediate stations.

In the example of FIG. 2, assume that the originating station 24originally heard an intermediate station 26 communicating with anotherstation 28, and therefore routes a message which it wishes to transmitto the station 28 via the station 26. If at that moment the connectivitybetween the stations 26 and 28 is lost, the station 26 may make anopportunistic decision to send the message via, for example, anotherstation 30, which has higher connectivity to the station 28. It will beunderstood that the alternate routing from the station 26 via thestation 30 to the station 28 is independent of the originating stationand is an opportunistic decision taken at the station 26. Similarly, ifthe station 30 finds that it cannot communicate directly with thestation 28, it too will opportunistically seek an alternative route, andmay have to relay its message via another station 32.

With the opportunistic relay techniques employed, there is no attempt tominimize the number of hops, but rather to maximise network throughputand the speed of message flow. Many hops may be dynamically andopportunistically necessary to achieve this optimum. Since at each hoprevert back checks avoid contention and overloading of any particularstation, and message time-out (time to live) and end-to-end messageacknowledgements prevent lost messages within the network clogging thesystem or never reaching their destination, as would be the case inflooding networks, the described network is extremely robust comparedwith fixed routing, adaptive routing or flooding algorithms.

As the above described network utilises a non-deterministic method ofoptimising itself and relies on adaptive feedback on a collective basis,no closed form method of predicting system capacity or delay time ispossible. The only method of determining these parameters is throughsimulation and exercising on a simulation basis to determine parameterswithin the network.

Since the network stations learn from past results and adapt tomonitored changing conditions, the flow of messages to themselves andamongst other stations, the monitored activity of other stations, andthe adaptive feedback of other stations, groups of stations routingmessages in the network can be considered as the co-operating decisionmakers of a team organisation. Each station has an artificialintelligence engine, which generates the routing variables andadaptation parameters. The parameters gathered from the monitoringengine and the long-term data base (see FIG. 13) play the role of thetraining patterns required for the artificial intelligence. The weightsof the various parameters within the artificial intelligence are thenadjusted and trained, based upon the dynamic changing parameters of thenetwork. Since the stations adapt to one another, the overall networkmay be considered as a greatly parallel-distributed processing systemwith the ability to configure routes for data flow and to adapt thetransmission power and other parameters of each station through dynamiclearning. This provides a near optimum flow of data across the networkand optimises network capacity.

The network may, alternatively, be considered as a greatlyparallel-distributed processing system with the ability to configureparameters such as transmission power, data rate, rate and duration ofsignal transmissions, through dynamic learning. This allows dynamicresponses to traffic conditions and changing propagation conditionsmeasured through out the network. The network can thus operate tooptimise message traffic requirements by adapting one or more parametersof operation.

Since the basic network protocol is very simple, requiring only twobasic message types and the adaptive feedback ability described above,even very simple artificial intelligence can be used to drive eachstation in small, low capacity networks. As a network expands, the"intelligence" of the stations can be upgraded without the necessity ofupgrading the basic link protocols. Since no routing information ispassed around the network, low and high "intelligence" stations can bemixed without compatibility problems.

Since the network is a co-operative network, the only level of servicethat can be "guaranteed" to users is that based upon the level ofpriority and the extent of the network. Where network traffic becomeshigh and delays increase, additional stations can be added to thenetwork, some being connected to more traditional high capacitynetworks, thereby maintaining overall message flow. However, the networkof the invention will never fail catastrophically, since there is nosingle point of failure such as a base station or controller node.

Different users may have different levels of priority. For example, someusers may have access to higher transmission power or high duty cyclesand the ability to introduce messages with a higher priority embedded inthem, as well as the ability to re-introduce messages more often, evenif end-to-end acknowledgments are not received. The described systemallows high priority and low priority users to be mixed in a commonnetwork.

Referring now to FIG. 7, this diagram is used to explain the probabilityof message flow via the network. At the originating station A themessages are entered and opportunistically wait for any station whichhas a high probability of routing a message to the destination stationO. Assume that stations closer to the destination have a higherprobability of communicating with the destination. The highestprobability of relay is from, say, the originating station A to astation B. Assume that peaks of opportunity exist between theoriginating station A and all of the stations B to O in an opportunisticenvironment, it is possible for the message to be routed directly to thedestination station O from the originating station A, but this has avery low probability.

From the first relay B, it is possible that the message can be sent toany of the stations G to O. Assume that the message is transmitted fromthe station B to a station I, based upon an opportunity. Again, thestation I might transmit to any of the stations L to O. Assume that thestation I transmits the message to a station M, the highest probabilityroute will then be to the destination station O itself.

Therefore, as the message is routed hop by hop, the number of stationswith a higher probability of communicating with the final destinationstation decreases, until at the final hop there is only one choice. Itis therefore an imperative in the network to make the number ofopportunities of intermediate hops as large as possible, and hop to apenultimate relay selected so that the last hop has a very highprobability of success. In the network, a higher probe rate and highergeneral network activity Will increase the number of opportunities, andtherefore the probability of finding an opportunity. As the message isrouted towards the destination, and the number of "choices" is reduced,the size of the hops must be reduced or, alternatively, the probe rateor level increased. This emphasises the importance of the "extra" hopsthat may be necessary to make the last hop one of extremely highprobability. Since the system is always looking forward 3 hops, it ispossible to ensure that the last hop has a higher probability.

Since the total probability of a message getting from the originatingstation to the destination station it is a product of the intermediateprobabilities, the objective of the network is to keep the probabilityof success of each hop as close to unity as possible. Equation 1 givesthe probability of success of a single hop: ##EQU1## where P_(i) is theprobability of transmitting to a station with some connectivity to theoriginating station and higher connectivity to the destination station,and where n is the number of stations.

The probability of each intermediate hop is a strong function of thenumber of "choices". Therefore, even if the intermediate hops have a lowprobability, the probability of finding any one of them is high.Therefore, large hops with low probability can be made for the firsthops, providing the last hops have high probability. In this case, thetotal probability which is a product of all the intermediate hopprobabilities, will be high. (See equation 2).

    P.sub.TOT =P.sub.HOP1 ×P.sub.HOP2 ×P.sub.HOP3  (2)

For example, low power stations with low connectivity between themselvescan route messages on behalf of each other towards destinations,providing there are sufficient of them. In the case of a vehicularnetwork, vehicles can relay messages between each other and towardsfixed dispatch centres which have higher power and duty cycle, and willprovide a high probability last hop to the destination vehicle oncemessages are routed close enough to them from other vehicles.

Similarly, in a utility environment where homes have low power, lowperformance radios, messages can be routed from home to home until themessages are sufficiently close to a data gathering or datadissemination station which has higher power and a higher duty cycle andwhich can ensure a high probability last hop.

Individual stations can "scavenge" messages opportunistically in orderto enhance connectivity. For example, if a first station iscommunicating with a second station, a third station, which hasdetermined that it is better placed to act as a relay between the firststation and the desired destination station, or that it can act as arelay between the first and second stations, can actively intervene toact as a relay, thereby allowing the first and second stations to reducetheir transmission power levels.

The priorities of the system can therefore be summarised as

1. making the number of options as high as possible;

2. ensuring that the probability of intermediate hops will collectivelyprovide a high probability;

3. ensuring that the message is routed to a final relay point which hasa very high probability of reaching the final destination; and

4. always routing messages towards stations with higher connectivity.

Referring now to FIGS. 8, 9, 10 and 11, the hardware of the station ofFIG. 1 is illustrated in greater detail. The prototype of the stationwhich is described below was implemented as a portable radio telephonetransceiver for use in a voice communication network. The prototypetransceiver is intended to be used as a vehicle-mounted unit and wasconstructed in a housing which can be mounted under the dashboard or inthe luggage compartment, for example, of a motor vehicle and which issupplied with 12 volt DC power from the vehicle electrical system.

It will be appreciated that the transceiver could be provided in aminiaturised battery powered for for use as a personal transceiver, orcould be used as a base station or a fixed point relay, for example,mounted on a tower or mast with a suitably efficient antenna.

The circuitry of the transceiver is built in a number of modules, whichcorrespond generally to the block diagrams of FIGS. 8, 9, 10 and 11. Inthis regard, FIG. 8 shows a transmitter module of the transceiver,comprising an adaptive power amplifier with an output power range fromminus 40 dBm to 70 watts, a frequency synthesiser MSK modulator withdual data rates of 8 kilobits per second and 80 kilobits per second,power control circuitry and power protection circuitry. FIG. 8 alsoshows power measurement circuitry and a receive/transmit attenuator ofthe transceiver.

FIG. 9 shows a receiver module of the transceiver, which includes a lownoise preamplifier, a mixer, two IF stages and two MSK demodulatorsoperating at 8 and 80 kilobits per second.

FIG. 10 shows the main nicroprocessor of the transceiver together withassociated interface and control circuitry, while FIG. 11 shows themicroprocessor together with a dual vocoder interface and other userinterface components.

Referring to FIG. 8, an antenna 100 is connected a low power sensingcircuit 101, a transmit/receive switch 103 and a forward and reflectedpower measurement circuit 161 to a power amplifier 145. The poweramplifier is fed by first and second driver amplifiers 142 and 144 froma buffer amplifier 140, which is in turn fed with the output of avoltage controlled oscillator (VCO) 139 which forms part of amodulator/synthesiser circuit. In this circuit, a synthesiser 138 runsat the transmit frequency (in the 45 to 50 MHz range) and is two-pointfrequency modulated, meaning that the frequency reference source 137 forthe synthesiser is modulated by the low frequency component in the datato be transmitted, while the VCO 139 is modulated by the high frequencycomponent of the data. The modulation is carried out by respectiveanalog switches 135 and 136 controlled by respective driver circuits 133and 134 which arc fed with data to be transmitted at the relevant datarates at which the transmitter operates. The result is a GMSK signalwhich is fed to the amplifier section of the transmitter.

When receiving, the operating frequency of the synthesiser 138 isshifted a short distance away from the receive frequency of thetransceiver, a distance just greater than the bandwidth of the widest IFfilter in the receiver. This is done by feeding the output of thefrequency reference 137, which operates at 10 MHz, into a divider thatcounts cycles and at the overflow value of the counter removes a cyclegoing into the synthesiser. This allows rapid shifting of thesynthesiser frequency on transmission, without the need forreprogramming of the synthesiser when the transceiver goes into receivemode, and avoids the delays that would be involved in reprogramming orrestarting the synthesiser when switching from receive to transmit modeand vice versa.

For successful implementation of a network employing transceiversaccording to the invention, it is important to control the transmissionpower so that it is adequate for the signal path conditions applying atany moment, but not excessive, which would result in unnecessary powerconsumption and interference between adjacent stations. Based on itsmonitoring of the channel in use, the processor circuitry of thetransceiver generates a power control signal via a power control circuit141 which is applied to a comparator circuit 143 comprising a gaincontrol circuit and a low pass filter. The comparator circuit 143compares the power control signal with a transmission power measurementsignal, and outputs a control signal which varies the gain of the seconddriver amplifier 144 to increase or decrease the transmission poweraccordingly. This circuit operates to adjust the transmission power tocorrespond to the power of signals received on the same channel, so thattransmissions take place at an adequate but not excessive power level.

The buffer amplifier 140 regulates the output level from the modulatorVCO 139 to a constant level, while the first and second driveramplifiers 142 and 144 are class B amplifiers, while the gain of thesecond driver amplifier 144 being controllable. The amplifier 145 is aclass C amplifier, and its current consumption is measured to provide anindication of the output transmission power of the transceiver. Thecomparator circuit 143 effectively provides a feedback loop whichadjusts the output power of the transceiver towards a setpointcontrolled by the power control circuit 141, and varies the output ofthe power amplifier from 100 mW to 70 W.

In order to increase the range of output power of the transmitter, acontrollable attenuator is switched into the output path when outputtransmission power lower than 100 mW is required. The attenuator canapply up to 60 dB attenuation in 10 dB steps. Thus, the overalltransmission dynamic range can be adjusted over a range of 100 dB. Theattenuator 102 comprises a ladder of resistors 200 which are arranged inthree groups 201, 202 and 203, with values calculated to provide anattenuation of 30 dB, 20 dB and 10 dB respectively. (See FIG. 12.) Theresistors are switched in and out of circuit by controllable switches204 comprising PIN diodes which are effectively biased on or off bycontrol signals from the processor circuitry of the transceiver.

Depending on the combination of attenuator sections which are switchedin or out, a maximum attenuation of 60 dB, in 10 dB steps, is possible.Thus, the output transmission power of the transceiver can be variedbetween -40 dBm and 50 dBm, with rapid switching between power levels.This allows the transmitter circuitry to output consecutive bursts ofdata at different power levels as required. The attenuator circuit 102is also used in input power measurement, since the power measurementcircuit 161 can not operate over a very large range, typically only 60dB. By adjusting the switched attenuator appropriately, the effectivemeasurement range of the power measurement circuit 161 is extended to120 dB.

Referring now to FIG. 9, the receiver module of the transceivercomprises a high Q bandpass fitter 104 which is connected to thetransmit/receive switch 103. The filter 104 has a bandwidth ofapproximately of 1.5 MHz and a low insertion loss. The filter 104 isfollowed by a low noise preamplifier 105 with a high dynamic range, theoutput of which is fed into a mixer 106 which forms part of a 10.7 MHzIF strip. The output of the mixer is fed through a bandpass filter 107to a first high-gain IF amplifier 108. The output of this amplifier isfed to first and second ceramic filters 109 and 110 which providebandpass filtering, centred on 10.7 MHz, of 150 kHz. A second IFamplifier 111 follows the filters 109 and 110, to compensate for theirinsertion loss. A further ceramic filter 112, with the samecharacteristics as the filters 109 and 110, follows the second IFamplifier 111, to further improve the selectivity of the receiver. Thisfilter also provides a time delay which is required for noise blanking(see below).

The output of the filter 112 is fed to a noise blanker circuit 113 whichis essentially a controllable switch controlled by the output of anamplifier 126 which provides a blanking pulse output and which is usedto "blank" noise pulses, with attenuation of 40 dB when open. The outputof the noise blanker circuit 113 is fed through a narrow band 15 kHzcrystal filter 114 which provides a selectivity of 15 kHz centred on10.7 MHz. The output of this filter is fed into a third IF amplifier 115which has sufficient gain to overcome the losses of the previous stagesand to provide sufficient output levels to drive the NE 615 FMintegrated circuit 116 which follows.

It can be seen front the above description that the IF strip of thereceiver module provides gain and selectivity in two bandwidthssimultaneously, namely 150 kHz and 15 kHz. This allows simultaneousmeasurement and demodulation within two different bandwidths and at twodifferent data rates. The use of parallel demodulation chains withparallel data synchronisation and demodulation of the data allowssimultaneous data to be demodulated from two different stations at twodifferent data rates, with one of the two being chosen, based uponopportunistic decisions.

The NE 615 FM integrated circuit 116 is used to implement a 455 kHz IFstrip. The device incorporates a mixer/oscillator, two limitingintermediate frequency amplifiers, a quadrature detector, a mutingcircuit, a logarithmic received strength indicator (RSSI), and a voltageregulator.

The output from the third IF amplifier 115 is converted in theintegrated circuit 116 to a 455 kHz signal which is fed through aceramic filter 117 having a bandwidth of approximately 15 kHz centred on455 kHz, and then amplified to provide an RSSI output signal. Thisamplified signal is passed through a second ceramic filter 117,providing further selectivity, and amplified again, providing an overallgain of 90 dB. This makes it possible to measure a received signalstrength in the range -130 dBm to -30 dBm, a 100 dB range. The range ofmeasurement is extended by 60 dB through the use of the switchedattenuator circuit 102 (described above) to provide a total measurementrange of 160 dB.

The integrated circuit 116 includes a Gilbert Cell quadrature detectorwhich operates in conjunction with a quadrature phase shifter 118 toprovide FM demodulation of the incoming minimum shift key (MSK) data.This data waveform is taken from the output of the quadrature detectorthrough a filter and is available as narrow band output data (8 kbps).The use of a quadrature detector provides a rugged and effective methodof demodulation which is immune to frequency offsets and phasedistortion and does not require carrier recovery time.

A wide band IF strip is fed from the output of the first 150 kHz ceramicfilter 109 and comprises a gain stage 119, the output of which is fedinto an FM IF integrated circuit 120 comprising an NE 604 chip. Thisdevice is a low power FM IF system incorporating two limiting,intermediate frequency amplifiers, a quadrature detector, a multicircuit a logarithmic received strength indicator and a voltageregulator. The integrated circuit 120 uses a 150 kHz ceramic filter 121to provide a wider bandwidth RSSI output signal, so that the receivermodule can make high dynamic range signal strength measurementssimultaneously in a 15 kHz and a 150 kHz bandwidth.

A quadrature phase shifter network 122 is provided to allow theintegrated circuit 120 to demodulate MSK data in a similar manner to theintegrated circuit 116.

Apart from the wide band data demodulation and signal strength detectionfunctions described above, the integrated circuit 120 is also used fornoise blanking of narrow band received data. This is done by detectingshort noise spikes which are common in low VHF bands and aresignificantly shorter than the data period. For example, in the 455 kHzIF circuit (block 116) data is detected at 8 kbps, corresponding to abit period of 125 microseconds. If a noise spike of, say, 12microseconds occurs, this will inject noise for only 10% of the bitperiod. If the noise spike is passed through the 15 kHz filter, thepulse duration will become approximately 60 microseconds, resulting insignificant distortion of a single data bit. The noise blanker thereforeattenuates such noise pulses before they enter the narrow band filtersof the 455 kHz IF stage.

If a typical noise pulse is passed through the 150 kHz filters 109, 110and 112 of the 10.7 MHz IF strip, the pulse duration will beapproximately 6 microseconds, and if it is removed before the 455 kHz IFstrip, it will have a negligible effect on the bit error rate of the 8kbps data.

For this purpose, a differential trigger 123 and a timer 125 generateshort pulses corresponding to the duration of the noise pulse once thelatter has passed through the various filters. Since the noise pulsesare typically only nanoseconds long, the duration of the pulses at theoutput of the filters will be set to approximately 10 microseconds andthe timer 125 is therefore set to generate 10 microsecond blankingpulses. This delay corresponds to approximately 10% of the period of asingle data bit.

A spike counter and level detection circuit 124 is provided to allow thedetection and counting of noise spikes, which information can be used asan adaptive feedback parameter to select the duration of datatransmissions, and the data repeat rate, etc. For example, if noisespikes are measured at a 100 Hz rate, the wide band data could betransmitted in bursts between noise spikes at 10 ms intervals, therebyachieving a significant improvement in performance.

The noise spike detection signal from the counter circuit 124 can beused together with bit error performance data and RSSI information toprovide a number of adaptive feedback parameters for use in theoperation of the transceiver.

Finally, a receive synthesiser 160 provides a local oscillator at 55.7MHz to 60.7 MHz, mixing down from the 45 MHz to 50 MHz RF frequency tothe 10.7 MHz IF. This synthesiser is required to hop in frequency, basedupon instructions from the main processor, independently of the transmitsynthesiser 138. The synthesiser can be programmed to hop in frequencysteps which correspond to channels having a bandwidth which is the sameas that of the narrow band data. Since the narrow band data is within a25 kHz channel, the synthesiser may be programmed to hop to any channelwithin the bandwidth 45 to 50 MHz in 25 kHz steps. This allows thereceiver to demodulate, within milliseconds, data on different receivechannels between 45 and 50 MHz.

The described transmitter/receiver modules lend themselves to frequencyhopping operation, with the transmission and reception channels beinghopped together or independently. It will be appreciated that othertransmission schemes, such as direct sequence spread spectrum (DSSS)operation, might be preferable in particular applications.

As each station transmits probe signals and monitors received signals,it switches from one frequency channel to another, recording informationas to which other stations are available on the various channels andnoting their identity, signal strength, frequency of transmission andduration of transmission. Apart from switching between transmission andreception frequencies, each station can also (where applicable) selectbetween different antennas which are optimised for different frequenciesor transmission directions, for example.

A group of stations may hop synchronously or semi-synchronously. Forexample, a group of stations relaying messages on behalf of one anothermay switch frequencies/channels as a group. The operation of the networkin the above described frequency adaptive manner can be considered as aform of slow frequency hopping, frequency scanning, frequency divisionmultiple access methodology.

Stations which provide a mix of coverage and capacity and are availableas opportune links for one another will tend to congregate on particularchannels, or hop channels together in a synchronous manner. Since thetransmission/reception frequency is one of the adaptation parameters ofthe apparatus of the invention, it can be changed as required usingprobe and revert back signals. For example, one station may requestanother station to move to another frequency to "meet" it there, toprovide an opportune relay link, or to reduce traffic on anotherchannel.

Adjustment of the transmission/reception frequency may also be employedin the case of high priority data, where a free channel can be clearedand high power, the data rate transmissions used directly between twostations, for example.

Certain frequencies may be used as congregation or meeting frequencieswith multiple hop, fall connectivity, where stations exchange smallamounts of information at lower power levels and high data rates,thereby minimising their on-air time and maximising the overall networkconnectivity and information exchange. If two stations are able toestablish connectivity through multiple hops on such a channel, they mayelect, by coordinating between themselves (and possibly one or moreintermediate relay stations) to change to an opportunistically selectedchannel, which has low noise, low interference and/or low traffic forboth the source, the destination and the relay stations. This type ofopportunistic frequency change will most often occur when it isnecessary to exchange large volumes of data, normally requiringincreased power levels to improve connectivity. If the stations areunable to connect with one another at a first chosen frequency, they canchoose another channel or return to the original calling channel tore-establish connectivity.

Thus, it will be understood that the frequency used between stations isadapted in the same manner as other parameters such as the transmissionpower, the data rate, or the timing of a transmission to match a channelpeak.

The combination of adaptive channel hopping in conjunction in withadaptive transmission power and adaptive data rates is an importantfeature of the invention. Opportunistic channel hopping can be used tofind quiet channels with low interference or noise, or to find a channelwith traffic on it in order to locate a particular relay or destinationstation. Therefore, in quiet networks the stations may tend tocongregate on a single channel, making efficient use of that channelthrough adaptive power and adaptive data rate transmissions. However, asthe traffic on the channel increases, stations can opportunistically hopoff the channel to adjacent channels to exchange large volumes of dataor to form subgroups. Individual stations seeking transmissionopportunities can hop between groups of stations operating on differentchannels, and in certain circumstances stations may hop together as agroup from channel to channel for the purpose of creating transmissionopportunities.

Since in general propagation conditions frequency selective fading andfrequency dependent interference occur, channel hopping createstransmission opportunities with different characteristics which, inconjunction with other time varying channel characterstics, effectivelyadds an additional variable into the opportunistic environment used bythe network.

The effect of the above described opportunistic frequency hoppingoperation of the network is that stations operating as intermediate orrelay stations may receive a message from a particular station on onechannel, and hop to a second channel to pass the message on efficiently.For example, an originating station may not know on which channel tofind the destination station, but through the probing process, a relaystation opportunistically takes the message from the originating stationand transmits it to the destination station, which it has recently heardon another channel. Establishing the channels which stations are usingis therefore a distributed function, with numerous stations scanningcontinually and assisting one another in finding which channels otherstations are on. If a station cannot find the destination station, andhas probed on a number of different channels, the message is passed on,allowing other stations to probe for the destination station on variouschannels.

The overall receiver allows simultaneous demodulation, synchronisationand capture of data at two different data rates with a high range ofdynamic signal differences. Although the described embodiment caters fortwo different data rates, it is possible to extend the concept to caterfor further parallel data rates, typically spaced by orders ofmagnitude. For example, in the described receiver, provision could bemade for data rates of 800 kbps, 8 Mbps and 80 Mpbs in addition to the 8kbps and 80 kbps rates. In a typical network, the highest data rateshould be chosen, based upon the spectrum allocation, to fill thecomplete spectrum allocation. Therefore stations can call each otheropportunistically and dynamically at any data rate, and all otherstations can monitor and demodulate the transmissions. Due to theisolation between the different data rates, in many circumstances astation will be able to demodulate the transmissions of two differentstations simultaneously, one at a higher rate and one at a lower rate.

Turning now to FIG. 10, the main microprocessor and modem interfacemodule of the transceiver are shown. The main microprocessor 149 is atype 386 EX chip with associated static and dynamic RAM 150 as well asseveral EEPROM's (not shown) which program the operation of the receive,transmit, interface and processing functions of the transceiver. Theprocessor 149 has an associated real time clock 148.

Via a main bus 205, the processor communicates with a main analog todigital converter 146, a main peripheral interface 147, and a high speedserial controller chip 131, which in the prototype transceiver was aZilog Universal Serial Synchronous controller chip.

Data to be received and transmitted is fed via the serial controller 131to respective encoder/decoders 128 and 130 and their respective GMSKmodems 127 and 129, operating at 8 kbps and 80 kbps. In the prototype,the modems were type FX 589 GMSK modems. Output data is fed from themodems 127 and 129 to a transmitter interface 206 controlled by theperipheral interface 147 and a power control circuit 132. The modemsinclude a lock input which is controlled by the processor, allowing themodems to search for and acquire signals quickly and then to be lockedon, thereby reducing noise and interference, in particular interferenceresulting from other stations. This lock feature allows the modems topick out stations under processor control, which is important to theoperation of the invention.

Incoming data from the receiver is fed via a receiver interface 207 tothe modems 127 and 129 and via the encoder/decoders 128 and 130 to theserial controller 131 for processing by the main processor 149. Thebroad and narrow band RSSI signals and the spike counter level signalsfrom the receiver module are fed via the interface 207 to the analog todigital converter 146 for processing by the processor 149.

Referring now to FIG. 11, the vocoder interface module of thetransceiver is shown. The module comprises dual vocoders 152 and 153which are utilised, in the described embodiment, to convert voicesignals to data for transmission between stations in a network. Thevocoder interface module essentially converts speech into a digitisedform, and then compresses and "packetises" it before passing it to theprocessor circuitry.

The vocoders used in the prototype transceiver were Qualcomm type Q4400vocoders. In operation, audio signals from a microphone 208 are fed viaa microphone amplifier 158 to first and second PCM modules 155 and 156which sample the audio data and convert it to a PCM format. The PCM datais fed to the data input of each vocoder and internally grouped togetherinto 20 millisecond frames (160 PCM samples per 20 millisecond frame).These frames are encoded into packets and output to the main processor149 every 20 milliseconds.

Each packet of compressed speech data is transferred to the processor ina TX frame response packet, which contains the data rate for the frameas well as the valid data bits. The processor determines the maximum andminimum data rate limits for the next 20 millisecond frame to beprocessed. Each packet of compressed data received by the processor 149is formatted for transmission, with a processing delay between arrivalof the first PCM sample in the 20 millisecond frame and the completionof the encoding process for that particular frame being approximately47.5 milliseconds. Once the frame of data is in the processor, theprocessor reads the data rate bit and strips out redundant informationbefore packetising the data and outputting it via the serial controllerchip 131.

Due to the opportunistic burst mode operation of the transceiver, it isimportant to overcome the potential disadvantage of having a lag in therate of adaptation of the voice signal, which could result in losingtransmission opportunities which happen in windows significantly smallerthan 20 milliseconds. This is why two parallel vocoders 152 and 153 areused, providing two (or more, if additional vocoders are used) optionsfor use by the main processor 149. For example, the processor 149 caninstruct the vocoders to operate fixed rates of, say, 4000 and 9600 bps,and select data from either vocoder according to a calculatedopportunity. Thus, the processor would have a choice of two differentdata packet sizes for transmission at each available transmissionopportunity. Alternatively, if one packet is sent at a higher data rateand is not successfully received, a lower data rate version of the sameframe, transmitted together with the subsequent frame, could be insertedinto the next packet that is transmitted. This provides a method ofbuffering of the transmitted data packets and effectively providing aform of opportunistic data rate transmission as well as a data durationadaptation feature.

It will be understood that more than two parallel vocoders could beemployed, operating at different data rates, with their output packetsbeing placed in parallel buffers available for opportunistictransmission. Those packets which are not transmitted, due totransmission of a packet from a different vocoder, are then simplyerased and replaced with subsequent packets.

Apart from operating at different data rates, the vocoders can be setwith different vox settings and different coding delays. Thus theprocessor could, for example, set one of the vocoders with a low voxsetting and a low data rate, with the other being set at a high voxsetting and a high data rate. This scheme can be used to ensure that thebeginning of a speech transmission is captured, with a switchsubsequently being made to a high data rate, high quality transmission.The provision of dual vocoders also allows one vocoder to be used todemodulate data while the other modulates data, therefore avoidingdelays in interactive speech, with one vocoder being opportunisticallyswitched over to pick up the beginning of a reply while the other isstill playing out the end of the received speech. This arrangementsignificantly reduces delays, particularly in an interactive situation.

Received data is sent to one of the vocoders 152 and 153, while theother receives blanked or erased data frames. Blanked frames are alsooutput when corrupted data is received, to prevent a distorted outputfrom the vocoder. In these circumstances, the vocoders interpolate orreconstruct the missing data. The received output from the vocoder 152or 153 is fed to the respective PCM module 155 or 156, with the audiooutput from the relevant module being selected by an analog audio switch157. The selected audio output is fed via a speaker amplifier 159 to aloudspeaker 210.

The overall adaptive rates of the vocoders are adaptively changed withinbroad boundaries due to long term feedback over seconds, andopportunistically changed within tens of milliseconds, based onselection of the buffered data frames for a number of parallel vocoders.The speech is replayed continuously on all the vocoders at a destinationstation, and using simple analog selection, the voice output is selectedfrom one of the vocoders. Timing is maintained between the parallelvocoder paths by the insertion of blanking and erasure commands to thosevocoders which have not received data packets.

The voice activated switch function of the vocoders is used todistinguish when a user is speaking. The decoder function will normallyhave priority over the encoder function. If both users at opposite endsof a link are speaking at the same time, the user at the far end willnormally be given priority. So-called "comfort" noise frames and theabove described blanking and erase commands are used to fill in gaps dueto packets which are lost in transmission, or packets which have beendelayed and are received out of sequence due to a multi-hop link. Thereceived speech can efficiently be speeded up by removing comfort noiseframes and slowed down through the insertion of blanking frames,allowing for a smooth flow of speech, despite the variable delay overthe link.

In order for a network to operate efficiently using the above describedtechniques, it is important that transmitted data packets be tracked, toprevent the clogging of the network with old data. The use of the realtime clock 148 allows each packet transmitted to be given a relativetime stamp, which is decremented as the packet is passed through thenetwork at a rate which is set relative to the real time. Packets whichare not successfully received by the intended destination station withina predetermined time-out period are deleted, preventing clogging of thenetwork.

Each station maintains a log of all the messages passing through it, toprevent messages travelling in a closed loop in the network. Once astation has passed a particular message on, it will in future, by revertback checks, prevent that message from passing through it a second time,and will simply redirect it elsewhere. Together with the above describedtime-out marking, this prevents messages from circling around uselesslyin the network.

FIG. 13 is a schematic diagram showing the overall software architectureof the transceiver, in a flow diagram form. The diagram summarises theabove described operation of a transceiver operating in a network ofsimilar transceivers.

It will be appreciated tat the embodiment of the invention describedabove is only one of many possible implementations of the invention, andshould be construed in a non-limiting way.

What is claimed is:
 1. A method of transmitting message data from anoriginating station (A) to a destination station (O) in a networkcomprising a plurality of stations (A to O), the methodcomprising:monitoring, at the originating station (A), the activity ofother stations (A to O) in the network; and transmitting the messagedata to at least a first intermediate (B) station for onwardtransmission to the destination station (O);characterised in that themethod further comprises the step of transmitting confirmation data backfrom the first intermediate station (B) to the originating station (A),indicative of the onward transmission of the message data,and in thateach station (A to O) in the network monitors the quality of the signalpath to other stations and in that the selection of the firstintermediate station (B) by the originating station (A) and theselection of any further intermediate stations (I,M) by the first or asubsequent intermediate station is made opportunistically, at the timeof the transmission of the message data, according to predeterminedcriteria including the monitored quality of the signal path between thetransmitting station and potential intermediate stations, so thattransmissions take place during peaks of opportunity.
 2. A methodaccording to claim 1 wherein each station in the network monitors theactivity of other stations on an ongoing basis in order to determine theavailability of those other stations, according to predeterminedcriteria, as intermediate or destination stations.
 3. A method accordingto claim 2 wherein the monitoring is carried out by receiving datatransmitted by the other stations, and analysing the received datatransmissions to select an intermediate or destination station.
 4. Amethod according to claim 3 including extracting information from thereceived data indicating at least the identity of the other stations. 5.A method according to claim 4 including extracting information from thereceived data relating to the destination and/or the origin of messagedata transmitted to or received from the other stations.
 6. A methodaccording to claim 5 including extracting information from the receiveddata relating to the final destination and/or primary origin of themessage data.
 7. A method according to claim 4 including extractinginformation from the received data relating to the propagation delay ofeach message, the data rate of each message and/or the volume ofmessages between any two or more stations.
 8. A method according toclaim 3 wherein data transmitted by each station includes time data, themonitoring including determining the age of data transmissions receivedfrom other stations in the network and discarding data transmissionsolder than a predetermined age.
 9. A method according to claim 8including comparing the time data in the received data transmissionswith a reference time, and discarding the received data transmissions apredetermined period after the reference time.
 10. A method according toclaim 8 including allocating a priority to received data transmissions,and adjusting the order of retransmission of the received datatransmission to other stations according to the age thereof.
 11. Amethod according to claim 1 including monitoring the quality of thesignal path between the first station and one or more of the otherstations and adapting, according to predetermined criteria, at least oneparameter of a subsequent transmission to another station in accordancewith the monitored quality of the signal path to increase theprobability of the transmission being received successfully.
 12. Amethod according to claim 11 including extracting information from thereceived data relating to the quality of the transmission path betweenany two or more of the other stations.
 13. A method according to claim 4including deriving from the received data adaptation information for usein adapting, according to at least one predetermined criterion, at leastone parameter of a subsequent transmission to another station toincrease the probability of the transmission being receivedsuccessfully.
 14. A method according to claim 13 wherein the adaptationinformation is transmitted to one or more other stations in anadaptation signal, the one or more other stations being responsive tothe adaptation signal to vary at least one parameter of a subsequenttransmission therefrom.
 15. A method according to claim 13 wherein theparameter which is adapted is one or more of the data rate, transmissionpower, transmission frequency, transmission or reception antenna,message length, message priority, message time to live, time oftransmission, and message retransmission rate.
 16. A method according toclaim 1 wherein the monitoring further includes transmitting a probesignal from the first station to at least one intermediate station, theprobe signal containing at least address data identifying the firststation, and transmitting an acknowledgement signal from the at leastone intermediate station to the first station.